In typical nuclear measurements devices, the nuclear detector is based on scintillating material. Scintillating materials produce light when exposed to nuclear radiation. The amount of light produced is related to the amount of ionizing radiation impinging on the scintillating material. In the case of gamma radiation, the spectrum of light produced is dependent on whether the energy is dissipated via Compton scattering or the photo electric absorption effect. Scintillation materials with an atomic number less than 25 are primarily subject to Compton scattering, while scintillation materials with atomic numbers greater than 25 are subject to Compton scattering and photo-electric absorption. Compton scattering produces a broad spectrum of light and there is generally no distinguishable characteristics or photo peaks. Conversely, the photo-electric effect produces a distinguishable photo peak, based on the energy of the absorbed gamma radiation.
Light is detected through the use of a Photo Multiplier Tube (“PMT”), which converts incoming photons to electric current pulses. A PMT coupled to an end of the scintillating material detects light emanating from the scintillating material. The PMT produces a signal indicative of the amount of radiation impinging on the material, which is representative of a particular measurement of the device. This type of sensor is discussed in U.S. Pat. Nos. 3,884,288, 4,481,595, 4,651,800, 4,735,253, 4,739,819, and 5,564,487, the entireties of which are incorporated by reference herein. Other nuclear radiation detection technologies have also been used in nuclear type detectors, e.g., a Geiger tube is shown in U.S. Pat. No. 3,473,021, the entirety of which is incorporated by reference herein. There are also scintillating detectors that make use of two different scintillating materials known as Phoswich detectors. A Phoswich (“phosphor sandwich”) is a combination of scintillators with dissimilar pulse shape characteristics optically coupled to each other and to a common PMT (or PMTs). Pulse shape analysis distinguishes the signals from the two scintillators, identifying in which scintillator an event occurred.
Unfortunately, there are several disadvantages with conventional ionizing radiation measurement devices, particularly those devices using scintillating material as a radiation detector. The gain of the PMT shifts with temperature and, in general, the light yield of the scintillation material usually changes with temperature as well. Other factors such as dark current pulses may be issues, but the primary disadvantage of a scintillation device is temperature related.
In the case of high atomic number (Z) scintillators that exhibit photopeaks, like NaI, methods to compensate for temperature effecting the PMT gain and scintillator light yield change due to temperature and based on tracking shifts in the photopeak spectrum are well known and readily available. However in the case of low Z, organic and/or plastic scintillation detectors, photopeaks, if present, are not distinguishable. Therefore temperature compensation and/or auto gain stabilization, based on the photopeak detection methods used for NaI scintillation, is not applicable to plastic or organic scintillation devices.
Plastic and organic scintillating materials generally have a relatively stable light yield over a temperature range from −60° C. to 40° C. However, the PMT gain shift over temperature is still enough of an issue to mandate temperature compensation. This temperature drift due to temperature changes may affect the gain by as much as one half of one percent per degree Celsius.
Contemporary methods of temperature compensation used to null these gain shifts are generally open loop, using a function that approximates light yield versus temperature and PMT gain versus temperature. Adjustments to the PMT gain are made based on this function. For example, a temperature reading may be taken and the electronic and/or high voltage gain may then be adjusted based on that reading. Additionally, other contemporary methods of temperature compensation may consist of shining a light emitting diode (LED) down the scintillating material. In an ideal temperature situation, a percentage of the light from the LED is detected on the other side of the scintillating object. As the temperature increases, however, less light is detected. A measurement is taken of the amount of the LED's light that was detected, and a compensation adjustment may then be made to the gain.
Therefore, there is a need in the art for a better methodology for gain control of the PMT based on the temperature dependence of both the PMT and scintillating materials.